The described embodiments relate generally to power converters.
Typically two-level power converters are used to convert wind turbine generated power into grid power. Two-level converters are cost effective at lower power levels. As power levels increase, a multi-level (herein meaning three or more) power converter embodiment is thought to be more cost effective.
One challenge for a three-level neutral point clamped converter topology is neutral point unbalance. For optimal operation of a typical three-level converter, the neutral point of the capacitor bank must be maintained at a voltage near the mid-point of the DC link. During normal operation, a three level converter pulse width modulation (PWM) strategy can be used to balance (center) the neutral point.
For PWM balance control, the basic principle is to inject an additional compensating signal during PWM modulation. By this technique, the neutral point current is regulated to charge or discharge the DC capacitors to compensate the capacitor voltage unbalance. Typical PWM balance control approaches include common mode signal injection using either carrier based modulation or space vector modulation.
During large grid disturbances, however, even when using PWM, the neutral point voltage may diverge away from the zero voltage potential. The neutral point voltage unbalance will increase voltage stress on the DC link capacitors and converter switches and result in a distorted output voltage waveform. For general-purpose applications such as motor drives, large grid disturbances will cause the converter to trip and thus avoid undue stress. However, wind turbines must be designed to ride-through large grid disturbances. Therefore, these conditions are of particular concern to wind turbine embodiments. In addition, any technique for balancing during these transient periods should be selected so as to minimize impact on output power quality and minimize component losses and stress.